1. Field of the Invention
The invention refers to a sliding-spool valve for controlling the hydraulic pressure at a working port (A) to values between a low pressure and a high pressure, for which a valve spool is accommodated in a bore of a housing in a linearly movable manner and which has a low-pressure port (T), a high-pressure port (P) and the working port (A), for which in the first position of the valve spool the working port (A) is connected to the low-pressure port (T), while in a second position of the valve spool the working port (A) is isolated from the two further ports (P, T), and also with a third position of the valve spool in which the working port (A) is connected to the high-pressure port (P).
2. Description of the Related Art
Such a sliding-spool valve for pressure control in a brake servo unit is already known from DE 39 12 936 A1. The cost-intensive production of this sliding-spool valve and the constantly existing leakage must be seen as being particularly disadvantageous. The valve spool and the housing as a rule are produced from steel and have to be hardened. For reasons of minimizing leakage, the diameter difference must be very small and the shape tolerances and position tolerances must be designed in an extremely strict manner. This creates high costs in production.
Therefore, the desire for the lowest possible leakage flows presents a particular technical challenge because in the case of the customary sealing by means of a metallically limited gap this has to be designed in the order of magnitude of a few micrometers. Such precision is achievable only with special materials such as ceramic or with the already-mentioned hardened steel. Residual leakage remains in the case of sealing via a gap, nevertheless.
The use of elastomer seals, which enable a leakage-free sealing of sealing gaps if the gap geometry remains unaltered, is indeed already known from machine hydraulics. In the case of the conventional sliding-spool valve, however, this does not apply to the variable sealing gaps on metering edges. All practical attempts to use elastomer materials on variable sealing gaps led to destruction of the corresponding sealing bodies, for example by shearing off on metal metering edges or as a result of intrusion into bores.